1. Field of the Invention
Embodiments of invention relates to electrosurgical systems for sealing tissue. More particularly embodiment related to a probe or jaw structure that utilizes a polymeric positive temperature coefficient of resistance (PTC or PCTR) material in a sensing surface for control of thermal interactions with engaged tissue in (i) thermal sensing interactions and (ii) in I2R current-limiting interactions with tissue.
2. Description of Background Art
Positive temperature coefficient (PTC) device are known in electronic industries and are used as low power circuit protectors, thermal sensors and as constant temperature heaters. FIG. 1A is an exploded view of a current-limiting device or thermistor 5 that has a polymeric PTC material 10 sandwiched between a pair of foil electrodes (12a and 12b) and packaged within an insulator 14 (phantom view). FIG. 1B is a schematic view of a prior art current-limiting device or thermistor 5 in a circuit diagram showing that heating of the PTC material can limit current flow to the load 16. FIG. 1C is a schematic view of a PTC device 25 that consists of a constant temperature heating element for heating subject material 26 in contact with the device. In other words, the device of FIG. 1C comprises a PTC heater material that conducts heat to the engaged subject material 26. The use of a PTC material as a heating element as in FIG. 1C was proposed in a surgical jaw structure in U.S. Pat. No. 5,716,366 to Yates et al.
In previous PTC devices, the polymeric PTC material consists of a crystalline or semi-crystalline polymer (e.g., polyethylene) that carries a dispersed filler of conductive particles, such as carbon powder or nickel particles. In use, a polymeric PTC material will exhibit temperature-induced changes in the base polymer to alter electrical resistance of the polymer-particle composite. In a low temperature state, the crystalline structure of the base polymer causes dense packing of the conductive particles (i.e., carbon) into its crystalline boundaries so that the particles are in close proximity and allow current to flow through the PTC material via these carbon “chains”. When the PTC material is at a low temperature, numerous carbon chains form the conductive paths through the material. When the PTC material is heated to a selected level, or an over-current causes I2R heating (Joule heating), the polymer base material thus will be elevated in temperature until it exceeds a phase transformation temperature. As the polymer passes through this phase transformation temperature, the crystalline structure changes to an amorphous state. The amorphous state causes the conductive particles to move apart from each other until the carbon chains are disrupted and no longer conduct current. Thus, the resistance of the PTC material increases sharply. The temperature at which the base polymer transitions to its amorphous state and affects conductivity is called its switching temperature TS.
As long as the base polymer of the PTC material stays above its TS, whether from external heating or from an overcurrent, the high resistance state will remain. Reversing the phase transformation allows the conductive particle chains to reform as the polymer recrystallizes to thereby restore multiple current paths (e.g., low resistance) through the PTC material.
Conductive polymer PTC compositions and their use as circuit protection devices are well known in the industry. For example, U.S. Pat. No. 4,237,441 (Van Konynenburg et al.), U.S. Pat. No. 4,304,987 (Van Konynenburg), U.S. Pat. No. 4,545,926 (Fouts, Jr. et al.), U.S. Pat. No. 4,849,133 (Yoshida et al.), U.S. Pat. No. 4,910,389 (Sherman et al.), U.S. Pat. No. 5,106,538 (Barma et al.), and U.S. Pat. No. 5,880,668 (Hall) and EP-730 282 A2 (Unitika) disclose PTC compositions that comprise thermoplastic crystalline polymers with carbon particles or other conductive particles dispersed therein. The disclosure of each one of these references is incorporated herein by this reference.
PTC devices are typically only employed in a passive role in an electronic circuit, and “switch” when a voltage spike overheats the polymeric material thereby causing its resistance also to spike. However, these devices do not consider the problem of rapid switching from a conductive to a resistive mode. There is a need for conductive polymer PTC compositions such as PTC composites which can switch in an extremely rapid, repetitive manner from a conductive to a resistive mode. There is also a need for PTC materials which have pixelated (localizable) switching across a surface of the PTC composition.